Some of the copiers, facsimiles, printers, printing presses or other image-forming devices utilize the method of electrophotography for forming images.
In electrophotography, an electrostatic latent image borne on a photoreceptor is subjected to visible image processing using a developer. The photoreceptor is also called a latent image bearing body that bears a latent image. The developer is, for example, a toner.
There are two types of developers. That is, a one-component type developer and a two-component type developer. The one-component type developer is itself endowed with electrostatic and electromagnetic properties and is transferred and adhered to the electrostatic latent image. On the other hand, the two-component type developer contains two distinct constituents, a toner and a carrier. Recently, both of the developers are made finer and finer to improve the image quality, dot reproducibility and sharpness.
The transfer unit transfers the visible image, namely the toner-developed image borne on the photoreceptor onto a medium. After such transfer, the image on the medium is fixed.
In an example of the fixing process, a single roller is set in direct contact with the toner-covered medium. The heat and pressure of the roller heats up and melts the toner. As a result, the toner penetrates into the medium. This method is called a thermal roller fixing system.
The thermal roller fixing system includes a fixing roller with a built-in heat source and a pressure roller provided opposite to the fixing roller. The surface of the fixing roller is covered with a medium release layer to prevent offset. This medium release layer is made of a medium release lubricant such as Teflon (registered trademark). The surface of the pressure roller is covered with an elastic layer made of silicone rubber.
The pressure roller is pressure fit to the fixing roller. The pressure roller has its elastic layer so deformed as to imitate the peripheral profile of the fixing roller. The combination of the pressure roller and the fixing roller constitutes a fixing nip.
The thermal roller fixing system is advantageous as compared to a belt-using fixing system. The reasons are as follows. That is, thermal roller fixing system very effectively makes use of the working of heat and pressure, and the fixing efficiency in the thermal roller fixing system is high. As a result, the thermal roller fixing system is being preferably used in image formation over the belt-using fixing system.
As explained above, when the toner is finer it improves the dot reproducibility and sharpness, therefore it is advantageous to use a finer toner. A finer toner in which a volume mean grain size is between 5 through 10 micrometers is know. However, during the manufacture of a toner having above-mentioned grain size, toner particles having still finer size are inadvertently generated in great number. The grain size of such toner particles is below 5 micrometers and they account for 60 through 80 number percent in ratio. Such toner particles will be collectively called micro-fine toner for convenience sake.
It is disadvantageous for the image-forming devices if the ratio of the micro-fine toner increases. Therefore, grain size of the toner is so adjusted that the micro-fine toner accounts for about 10 number percent in ratio.
However, there is an increased demand for lower costs and resource conservation involving toner. The manufacturing process becomes complicated if a toner having almost uniform grain size is to be manufactured by removing micro-fine toner from the toner. This also increases the manufacturing cost. Moreover, the micro-fine toner separated from the toner is generally disposed so that there is a loss of resources. As a result, there is a growing demand that, it should become possible to use a toner as it is even if the micro-fine toner is mixed in it in a specific ratio. Concretely, there is a growing demand that, a toner having a volume mean grain size of from 5 through 10 micrometers and a grain size not larger than 5 micrometers accounting for 60 through 80 number percent can be used in the image-forming devices. Hereafter such a toner will be called a fine toner.
If a visible image is formed using a finer toner that contains micro-fine toner, the toner is liable to become liberated under the influence of static electricity. In the image-forming devices, the fixing roller and the pressure roller are interlocked so as to rotate during the image forming operation. As a result of friction between the rollers, there is generated an electrical charge of the order of 0 through minus 5 KV. During the image forming operation, the toner is fixed to the medium only because of the static electric force. However, since there is the electrical charge generated due to friction between the rollers, the toner fixed to the medium gets separated from the medium, becomes airborne, and finally gets adhered to the fixing roller. Take an example of a digital copier where a toner-developed visible image is negatively charged at a low value and the visible image transferred on the medium is attracted and adhered thereto by the positive electrostatic force induced onto the medium side.
As explained above, the surface of the pressure roller in the fixing apparatus is provided with a Teflon or other insulating layer. The Teflon insulator constitutes an extensive, negatively charged portion for which very reason, on entering the fixing apparatus, the toner-developed visible image is subject to the repulsion between itself and the large negative electrostatic force of the pressure roller. Should the visible image be thereby repulsed and get adhered onto the fixing roller, offset ensues.
Occurrence of this phenomenon is not limited to the finer toner alone. Such phenomenon is observed in conventional toner with a volume mean grain size not falling below 20 micrometers. Moreover, it is also noticed in the toners with the volume mean grain size reduced to around 5 through 10 micrometers and from which micro-fine toner has been removed.
It has already been confirmed that offset can be rendered inconspicuous if the surface resistivity of the pressure roller is made as lower as ranging from 1×1010 through 1×1012 Ω/square, and the ratio of micro-fine toner is low compared to conventional toner. However, it was confirmed with experiments that, offset did occur in case of the finer toner, despite the surface resistivity of the pressure roller having being lowered to around 1×1010 through 1×1012 Ω/square.
On the other hand, in order to prevent this offset, a technique of rendering the pressure roller surface electroconductive (which in terms of surface resistivity being around 1×102 through 1×104 Ω/square) has often been attempted.
In the case of conventional toner with a volume mean grain size of not less than 20 micrometers, the toner itself with substantial weight is rendered electroconductive and as the toner enters the fixing apparatus the electric charge initiates an electrostatic discharge, during which period the toner retains its own weight and so offset is reduced. By contrast, for finer toner, as soon as a visible image enters the fixing apparatus, the visible image is electrically shocked by the abrupt discharge of electric charges between the visible image on the medium and the pressure roller positioned just behind the medium. Because of this electric shock, the toner (micro-fine toner, particularly) on the image surface becomes liberated towards the fixing roller, hence offset occurs as a result. These phenomena get significant when use is made of the kind of paper (medium) having high resistivity that tends to boost the amount of electrostatic charge (including the case as at a second side fixing where the moisture content of the medium is more or less found lost in the fixing step of a first side in a two-sided copying mode).